Horizontally align objects in front or side 2D views
Snap Shift+S the cursor to vertices that will align it where you want the object to be (e.g., four corner vertices that are equally spaced apart vertically and/or horizontally).
Snap Shift+S the object to the cursor position.




Note: this assumes that the origin is at the center of the single plane. If not, you could set the origin to the center with Shift+Ctrl+Alt+C (origin to geometry) or go into edit mode, select the vertices you want to move and do Shift+S (selection to cursor (offset)).